DEFORMATION STRUCTURES IN TI-RICH TIAL POLYSYNTHETICALLY TWINNED CRYSTALS

Room-temperature deformation structures in Ti-rich TiAl polysynthetically twinned crystals, which contain only a single grain with the two-phase TiAl/Ti3Al lamellar structure, have been studied for both the easy (shear parallel to the lamellar boundaries) and the hard (shear across the lamellar boundaries) types of deformation by transmission electron microscopy. For both types of deformation, numerous deformation twins of the {111} [112BAR] type and ordinary dislocations with b = 1/2[110] were observed. A few superlattice dislocations with b = 1/2[112] were also observed but those with b = [101] were hardly observed. Although these features are common to the two types of deformation, a clear difference between the two morphology of ordinary dislocations was observed. Ordinary dislocations introduced by the easy type of deformation do not align along any crystallographic directions and tend to cross-slip to form three-dimensional tangles while those introduced by the hard type of deformation tend to align along the screw orientation. Based on the results obtained, the deformation mechanisms of Ti-rich TiAl compounds with the two-phase TiAl/Ti3Al lamellar structure were discussed, taking into account the existence of six types of TiAl ordered domain in the lamellar structure. Deformation twinning was concluded to be the principal deformation mode while slip along the [110] direction is the complementary mode to deformation twinning, and their operation depends on the crystal orientation of TiAl domains.